Superconducting tunnel junctions (STJs) are used for detecting individual particles such as photons. Examples are described in “Quasiparticle trapping in a superconductive detector system exhibiting high energy and position resolution”, Kraus et al, Physics Letters B, Vol. 231, No. 1,2, November 1989, pages 195–202; and “Superconducting particle detectors”, Booth and Goldie, Supercond. Sci. Technol. 9 (1996) 493–516.
Superconducting quasiparticle sensors such as STJs are typically fabricated on substrates and a problem can arise when incident photons or other particles such as neutrons, muons, ions or molecules of sufficient energy interact with the substrate to produce “substrate events” which modify the detected response.
The presence of substrate events in the detector response has always been recognised as a serious source of spectral contamination for thin-film low temperature detectors. In general, the volumes of the sensing elements of these detectors are limited by technological constraints (such as the difficulty of processing thick films) or application requirements (such as energy resolution or counting rate). A compromise is often required so that the detectors have less than unity quantum efficiency. This means that some fraction of the incident particle (photon) flux always interacts in the supporting structure of the detector. Even for a very thick detector, products of natural radioactivity or cosmic rays will generate spurious pulses due to interactions in the substrate. Some devices, particularly transition edge sensors, utilise thin membrane-type support structures for which interactions in the substrate are relatively rare. Measurements still indicate a low level of substrate artefacts from these interactions. Most STJ detectors are fabricated on thick, crystalline dielectrics. These allow the fabrication, for example, of epitaxial films giving increased quasiparticle diffusion coefficients as compared with disordered polycrystalline films, and hence increased detector count-rates and energy resolution. Fabrication on a robust substrate also permits high packing-densities for arrays of sensors.
The photons whose energies are to be determined by the detector are absorbed exponentially as a function of thickness. This means that for finite absorber thicknesses some fraction of the incident photon flux must pass through the detector and interact in the supporting structure. Some fraction of this absorbed energy is then able to couple back into the detector. The ultimately detected energy is inevitably less than the full energy of the incident photons so that the spectrum of detected energies shows artefacts that are unwanted and these make detection of broad-band incident photons with good energy resolution difficult.
A number of schemes have been proposed previously to reduce this source of spectral artefact. These schemes include deposition of one or more thin layers between the substrate and the superconductor, (see M C Gaidis et al, “Superconducting Nb—Ta—Al—AlOx-Al tunnel junctions for X-ray detection”, J. Low Temp. Phys. 93, 605 (1993) and A Poelaert et at, “The suppression of phonon induced noise in niobium superconducting tunnel junction detectors”, J. Appl. Phys. 79, 2574 (1996)), or even deposition of a thin normal-metal layer between the absorber and the substrate (see G Angholer et al, “Energy resolution of 12 eV at 5.9 keV from Al-superconducting tunnel junction detectors”, J. Appl. Phys. 89, 1425 (2001)). Although the latter approach is attractive, any benefit of film epitaxy is lost and an additional insulating layer is required to electrically isolate the superconductor from the normal-metal phonon barrier. Interactions will still inevitably occur in this electrical isolation layer.